Generally, active mounts are known which include a spring element and an incorporated powered actuator. In these devices, an input signal from a sensor is processed and then fed to a powered actuator to control the vibrational dynamics between a support and a vibrating body with the goal of reducing the vibration transmitted from the vibrating body to the support. Such active systems are taught in U.S. Pat. No. 2,964,272 to Olsen, U.S. Pat. No. 3,566,993 to Leatherwood, U.S. Pat. No. 4,796,873 to Schubert, U.S. Pat. No. 5,052,510 to Gossman, and U.S. Pat. No. 5,067,684 to Garnjost. These systems are useful for adding dynamic forces at various operating frequencies and can effectively cancel vibration emanating from a vibrating device, such that little or no vibration is transmitted. Many types of actuators have been developed for these active mountings as exemplified by U.S. Pat. No. 4,600,863 to Chaplin et al., U.S. Pat. No. 4,669,711 to Beer, U.S. Pat. No. 4,693,455 to Andra, and U.S. Pat. No. 4,699,348 to Freudenburg.
The concerns regarding the implementation of active mounts have revolved around system stability and isolator effectiveness. The conventional approach has been to take a signal indicative of motion from a support that is desired to be isolated from a vibrating device. This signal is then minimized via a control algorithm such as Least Mean Square (LMS), frequency-shaped control, or H-infinity control. However, in order to design the control system, the dynamics of the structure, and in some cases the dynamics the vibrating element must be known fairly precisely. In systems using these control signals to accommodate multiple resonant frequencies in the control band width, the algorithm and the electronics required to provide proper control become extremely complicated and expensive. With active mounts installed in vibrationally dense or modally dense environments, adding energy to the active mount at a frequency which coincides with one of these modes of the engine or support, can cause the system to become unstable and, rather than improving vibration isolation, can diminish it. Thus for these dynamically dense systems, it is important to determine and account for these modes.
The prior art devices have attempted to recognize these modes and, either, avoid imparting energy at those resonant frequencies or, implement a control system which in some way accommodates for these modes of vibration through proper phasing of input forces. The problem with these "state of the art" methods is that they become exceedingly complex when trying to account for the vibrational modes of the structure and the vibrational modes of the, source. In order to accommodate this type of control and its complexity, controllers containing complicated processors have evolved.
Although these complicated processors may be warranted in some applications, in others, they are impractical and cost prohibitive. Other prior art devices have attempted to simplify the complexity. U.S. Pat. No. 4,638,983 to Indigkeit et al. describes one such device wherein an active mount 1 includes a liquid filled hollow space, whereby the fluid is moved by a positioning element 11 which is actively controlled by a control unit 14. In Indigkeit et al. '983, a sync signal is utilized to establish the frequency of operation. Another such device is taught in U.S. Pat. No. 5,133,527 to Chen et al. wherein an active mount takes a signal from a sensor 10, processes it and synchronizes it with a tachometer signal. However, in many applications, the cost and complexity of these controllers using sync signals cannot be tolerated. Further, such tonal controllers as taught by Indigkeit et al. and Chen et al. would be incapable of producing meaningful results with a broad band vibrational disturbance. Therefore, there is a need for a cost effective and efficient active isolator capable of broad band vibration isolation.
U.S. Pat. No. 5,011,108 to Chen et al. describes an active mount where a transducer senses force acting on the mount and the signal is processed, passed through a band pass filter and then through a series of adjustable gain amplifiers and, finally, into an electromagnet. However, despite the apparent simplicity, the device is still too complicated for some applications and is constrained by the fact that there must be alignment between the magnet and the coil for proper functioning.